use zmq.PROBE_ROUTER to register connections to schedulers

[minrk]

There is a race where engine connections to schedulers takes a finite amount of time, and it is possible for clients to be informed of an engine's registration before the engine is fully connected to all schedulers.

The current registration design effectively assumes heartbeats will complete after registration is complete, but the faster launch-then-use nature of the cluster Python API reveals the race.

Using the zmq.ROUTER_PROBE should allow us to delay registration notification until all probe messages have been received by the schedulers.

[minrk]

This should be not only more rigorous, but faster because we can wait for the actual event: everything is connected, rather than needing to set a long heartbeat and/or registration timeout